In the prior art, color television shadow masks are used inside picture tubes immediately adjacent the front surface to ensure that the electron beams fall precisely on the desired phosphor dots to create a sharp picture. As the design of color television picture tubes has become more sophisticated and exacting, greater demands have been made on the performance of the shadow mask. However, the prior art procedure for producing a shadow mask is simply incapable of meeting these demands in an adequate manner and the result has been a large percentage of rejections in the finished masks. This means a loss not only of the finished shadow mask but also its associated picture tube face plate since the phosphor pattern in the tube is produced from one individual mask which must accompany the picture tube along the entire assembly line.
The prior art method for producing shadow masks comprises the use of cold rolled sheet steel which is covered with a resist material to protect and save the desired metallic areas. A pattern of holes is left exposed and removed with an acid spray so that a very delicate and almost transparent sheet of metal remains. This delicate sheet of metal must then be drawn and formed into the shape of the front of a picture tube and then annealed to relieve stresses. The forming process yields unpredictable results and distorts the hole pattern in a unique way each time. Accordingly, to ensure that the phosphor dots on the face of the picture tube are in the correct positions relative to the shadow mask, the phosphor is applied to the picture tube with a light activation process that employs the shadow mask itself. Each of the three colors of phosphor must be applied separately requiring the shadow mask to be mounted and unmounted from the picture tube three times. Thereafter the shadow mask must accompany the picture tube throughout the manufacturing process because only the shadow mask that was used to establish the phosphor dots will be suitable to direct the electron beams properly onto the phosphor dots during operation. Thus, if a defect shows up in the shadow mask it must be rejected and the picture tube face plate that was manufactured from it is rejected as well.
There are numerous reasons why defects show up in the prior art shadow masks. The steel sheet that is used is often nonuniform in thickness and may have impurities that show up as luter lines only after the shaping and annealing steps have been completed. Since the etched steel is fragile, it is often deformed in the shaping process causing the dot pattern to be displaced an unacceptable distance. The holes or slots are irregular in size in many cases and the mask can tear during the shaping process. Also, as is the nature of steel, it will spring back slightly from the forming position in an unpredictable amount causing variations in size and shape.
The prior art annealing process is fraught with difficulties in that oil canning often results producing ripples and buckling in the surface of the mask. Creating holes by etching with acid is not the best approach since it necessarily entails rounded corners in the holes and irregular edges. This causes poor fidelity in the television picture as a result of poor definition of the electron beam by the shadow mask. All of these problems lead to an unacceptably high rejection rate which causes large cost increases in the production of television picture tubes. My invention, however, overcomes all of these problems with an entirely new and novel approach to the production of shadow masks.